This invention relates to a system, including both apparatus and a method, for purifying air by irradiating a stream of air selectively with ultraviolet light having a wavelength short enough to transform molecular oxygen (O2) into ozone (O3) or with ultraviolet light having a longer wavelength capable of destroying ozone so that it returns to stable molecular oxygen or with ultraviolet light having both wavelengths at the same time.
Ozone is an allotrope of oxygen that can be formed when molecular oxygen, such as the oxygen in the air, is irradiated by ultraviolet light having a relatively short wavelength between about 100 and 200 nanometers (nm.). This type of ultraviolet light, which is sometimes referred to as very-ultraviolet light, or VUV, is typically produced by evacuated ultraviolet generators, the radiant energy of which breaks stable molecular oxygen (O2) into atomic oxygen (O), some of which then reforms as ozone (O3).
Ozone can also be produced by subjecting air to an arc discharge of sufficient energy, but one of the disadvantages of doing so is that such an arc discharge can also ionize nitrogen, resulting in the production of compounds of nitrogen that are undesirable and even dangerous. VUV, on the other hand, does not ionize nitrogen in the air and, therefore, does not create those undesirable nitrous and nitric compounds.
Ozone is a powerful oxidizer because it is not a stable molecule. It has a half-life measured in hours, and immediately after being formed, ozone molecules begin to return to the stable, molecular state by releasing the third oxygen atoms which then enter into destructive reactions with contaminants in the vicinity. Some of the ozone breakdowns take place close to the VUV source, but other ozone molecules travel with any currents in the air as the ozone spreads out from the place it was formed. Additional reactions take place at locations increasingly remote from the origin.
The use of ozone in a room is desirable in terms of sterilizing or sanitizing or purifying the room. “Sterilizing” means to destroy all pathogens, i.e., anything that can grow biologically. “Sanitizing” means to reduce the level of pathogens down to some acceptable minimum level. “Purification” means to destroy pyrogens as well as pathogens. For example, ozone will break down alcohols and hydrocarbons as if they were being burned, i.e., converted to carbon dioxide.
One of the great advantages of ozone as a sterilizer is that it is not selective in the reactions it initiates. It renders harmful hydrocarbons and pyrogens (reactive molecules) harmless by oxidizing them, and it destroys pathogens (microorganisms), either by reducing or destroying them or by cell lysing or oxidation.
But the fact that ozone is not selective means that it will also react with humans and other mammals, and such reactions can be dangerous, even for short exposure if the levels of ozone are too high. They can also be dangerous if the exposure is too long, even if the levels of ozone are relatively low. Caution must be observed in the use of high concentrations of ozone in rooms and other closed spaces that are presently occupied by humans and other mammals or will be occupied before the concentration of ozone has been reduced to a sufficiently low level by breakdown of the ozone molecules. In the case of spaces to be occupied by humans, the Food and Drug Administration (FDA) has set certain time and concentration limits for exposure to ozone. The way the standards are set up is, it is appropriate to put the ozone into a room long enough at adequate concentration to sterilize the room and then to remove it.
In addition to destroying pathogens and pyrogens and oxidizing harmful hydrocarbons, another effect of ozone generally favorable to humans is that its release into a space typically causes insects to vacate that space.
Although the instability of ozone causes it to return to molecular oxygen, it takes a matter of hours for half of the ozone molecules to break down and if the concentration is high, it may take a long time for a room to become habitable by humans unless the process is speeded up. One method of doing so is by passing the ozone-containing air through reactive media, such as an activated carbon filter. This requires that the air be put into motion as an airstream through the filter. The ozone will be destroyed by reaction with the media, and the rate at which that takes place can be increased by raising the temperature of the reactive media.
Another method of destroying ozone in an airstream is to irradiate the airstream with ultraviolet light at a wavelength in the range between about 200 nm. and 450 nm., particularly 254 nm. Radiation in that part of the ultraviolet spectrum is referred to in the following description as UV. UV radiation increases the rate of disassociation of atoms in the ozone molecules and accelerates reaction of the ozone and its atomic oxygen components with airborne contaminants.
Ultraviolet light, itself, in any part of the ultraviolet spectrum has long been known to have a germicidal effect in clearing the environment of certain pathogens but is more limited than ozone in the types of air-borne contaminants it will attack. For example, ultraviolet light, alone, will sanitize and sterilize the air for bacteria, but not so much for certain pathogens, such as fungus and molds, nor will it attack pyrogens. Furthermore, ultraviolet light travels only in straight lines from its source and can only attack those pathogens that are along a line of sight from the source. As a result, even those pathogens that would be destroyed if they were irradiated by ultraviolet light will not be destroyed if they happen to be shielded by any opaque material, even a shield as small as a microscopic dust particle. Ozone, on the other hand, can circulate throughout a space into which it is propelled by an airstream and thus can react with contaminants in any part of that space. The use of UV is a very effective way of sanitizing air, but not totally sterilizing it.